How does ionisation energy affect ionic bonding?
Ionisation energy is the amount of energy, which is required to remove the most loosely bound electron(s) from an isolated gaseous atom to form a positive ion.
In forming an ionic bond, one atom must form a cation by losing one or more electrons. In general, elements having low ionisation energies have a more favourable chance to form a cation, thereby having a greater tendency to form ionic bonds.
Thus, lower ionization energy of metallic elements favours the formation of an ionic bond. It is because of low ionization energy that the alkali and alkaline earth metals, form ionic compounds.